The nuclear industry is expanding, and the heat-treating community needs to keep pace. This article provides a brief overview of nuclear power, discusses the sintering applications and takes us into the future of the industry.
Editor’s note: This is the second of three articles, which are also supported by online-exclusive facts and data. As a result, figures and tables are numbered consecutively whether they are included in print or not. If you are seeking a figure not in print, find it in the online exclusive at www.industrialheating.com/nuclear610.
June’s article and online exclusive provided much more detail about the nuclear power industry. Please refer to them for an in-depth description of the types of reactors and heat-treating applications.
|Fig. 5. Nuclear fission reaction|
Brief Overview of Nuclear Power
To provide the power for an electric generator, nuclear power plants rely on the process of nuclear fission. In this process, the nucleus of a heavy element, such as uranium, splits when bombarded by a free neutron in a nuclear reactor. The fission process for uranium atoms yields two smaller atoms, one to three free neutrons and an amount of energy (Fig. 5). Because more free neutrons are released from a uranium fission event than are required to initiate the event, the reaction can become self-sustaining, creating the familiar chain reaction under controlled conditions and yielding a tremendous amount of energy.
In the vast majority of the world’s nuclear power plants, heat energy generated by burning uranium fuel is collected in ordinary water and carried away from the reactor’s core either as steam in boiling-water reactors (BWR) or as superheated water in pressurized-water reactors (PWR).
Boiling-water and pressurized-water reactors are so-called light-water reactors because they utilize ordinary water to transfer the heat energy from reactor to turbine in the electricity generation process. In other reactor designs, pressurized heavy water, gas or other cooling media transfer the heat energy.
Sintering Nuclear Fuel Pellets
Military development programs led to the introduction of ceramic-oxide fuels in the 1950s. Today, practically all fuel is ceramic (Table 6) – either urania (uranium dioxide, UO2) and/or plutonia (plutonium dioxide, PuO2). These materials have unique features that qualify them for nuclear fuel applications. These features include:
- They are extremely stable refractories. For example, the melting point of UO2 is in excess of 2800°C (5072°F).
- The open crystal structure allows for retention of fission products.
- Their highly variable oxygen-to-metal ratio can shift to accommodate burn-up.
Therefore, they have excellent resistance to radiation damage. Other advantages include inertness to many coolants, long burn-up without swelling and relatively low fabrication cost. One drawback, however, is low thermal conductivity. This has prompted research on replacing the oxides with more conductive carbides or nitrides (Table 7).
The fuel pellets, usually on the order of 1 cm (0.40 inch) diameter x 1.5 cm (0.60 inch) long, are arranged in a long zirconium alloy (zircaloy) tube to form a fuel rod. Up to 264 rods form a fuel assembly, which is an open lattice and can be lifted into and out of the reactor core. These are about 3.5-4 meters (11.5-13 feet) long in the most common reactors.
The fabrication of ceramic nuclear fuels traditionally follows a standard powder-pellet process. This involves comminution, granulation, pressing and sintering in a reducing atmosphere. The pores are intended to retain fission gas and decrease swelling during burn-up.
The Sintering Process
Sintering of UO2 and PuO2 fuel pellets significantly increases their density from 5.5-6.0 g/cm3 in the green state to 11-14 g/cm3 as sintered. Sintering converts UO2 from a green, compacted powder to a ceramic with the microstructural and physical properties necessary for service at high temperature in a reactor core. Small voids (porosity) are removed as the particles shrink and coalesce into a crystalline grain structure while maintaining the overall shape of the pellet. The resulting microstructure consists of large, equiaxed grains with uniformly distributed spherical pores on the order of 2-5 micrometers (0.00008-0.0002 inch).
Sintering takes place by solid-state diffusion in the temperature range of 1680-1800°C (3060-3270°F) for a period of up to five hours. Walking-beam (Fig. 26) and pusher-style furnaces are common, being electrically heated with high-purity alumina refractory brickwork in the high-temperature zones. Pellets are placed in molybdenum boats (trays). To prevent oxidation of the UO2, a reducing atmosphere of hydrogen, hydrogen/nitrogen or dissociated ammonia is used. Some moisture within the sintering-furnace atmosphere is desirable to prevent degradation of the refractory, but oxidation of the molybdenum heating elements must be avoided.
|Fig. 27. European pebble-bed reactor concept (one of six classes of Generation IV initiative programs)|
Wet grinding of the pellets is necessary after sintering due to shrinkage. The type of wheel used (silicon carbide, diamond) dictates if further cleanup is required. An annealing process follows grinding and improves mechanical strength of the pellets.
|Fig. 28. Fuel source for pebble-bed reactor|
The Future of Nuclear Power?
In Europe, as elsewhere around the world, development is under way on a Generation IV reactor design with six styles under consideration. One of these is a pebble-bed reactor (Fig. 27) with its unique approach to fuel-pellet design (Fig. 28). The pebble-bed reactor (PBR) is a graphite-moderated, gas-cooled nuclear reactor. It is a type of very high-temperature reactor (VHTR) formally known as the high-temperature gas reactor (HTGR), one of the six classes of nuclear reactors in the Generation IV initiative. The PBR achieves high outlet temperatures.
The base of the PBR’s unique design is the spherical fuel elements called “pebbles.” These tennis-ball-sized pebbles are made of pyrolytic graphite (which acts as the moderator), and they contain thousands of micro-fuel particles called TRISO particles. These TRISO fuel particles consist of a fissile material (such as U235) surrounded by a coated ceramic layer of SiC for structural integrity and fission-product containment.
In the PBR, 360,000 pebbles are amassed to create a reactor core and are cooled by an inert or semi-inert gas such as helium, nitrogen or carbon dioxide.
This type of reactor is also unique because its passive safety removes the need for redundant, active safety systems. Because the reactor is designed to handle high temperatures, it can cool by natural circulation and still remain intact in accident scenarios, which may raise the temperature of the reactor to 1600°C (2910°F).
Because of its design, its high temperatures allow higher thermal efficiencies than possible in traditional nuclear power plants (up to 50%), and it has the additional advantage that the gases do not dissolve contaminants or absorb neutrons as water does, so the core has less radioactive fluids. A number of prototypes have been built, and active development is ongoing.
|Fig. 29. Hyperion power module (HPM)|
The idea of compact nuclear power-plant designs (Fig. 29) is generating some interest in the U.S. As part of Los Alamos National Laboratory’s technology transfer program, a license has been issued to Hyperion Power Generation to commercialize a nuclear fission reactor apparatus (U.S. Patent Application 20040062340).
The method for operation comprises a core that is a fissile metal hydride; an atmosphere comprising hydrogen or hydrogen isotopes to which the core is exposed; a non-fissile hydrogen-absorbing and desorbing material; a means for controlling the absorption and desorption of the non-fissile hydrogen-absorbing and desorbing material; and a means for extracting the energy produced in the core.
The concept is to bury these modules underground and guard them with a security detail. Like a power battery, these modules have no moving parts to wear down and are delivered factory sealed. Each unit is intended to produce 27 MW when connected to a steam turbine, which is enough energy to provide electricity for 20,000 average U.S. homes. They are never opened on site and cannot go supercritical, or “melt down.” The waste product produced after five years of operation is about 40.6 cm (16 inches) in diameter and can be used for fuel recycling. Even if compromised, the material inside is inappropriate for proliferation.
The U.S. Nuclear Regulatory Commission (NRC) has yet to review the design since a license application has not been submitted. Currently, work continues on product development, evaluating alternative materials and design features, and executing strenuous tests. IH
The Nuclear Renaissance online exclusive originally posted in June contains the figures missing from this printed article as well as more detail on nuclear power generation. Go to the article by clicking here.
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